The MagSafe 2 was necessary for the form factor of the Retina MacBook Pro, not the Air. Apple carried the new design over to the Air to jump start the process of replacing old tech with new tech, just as they updated the iPad and iPods to Lightning ports shortly after introducing the new port on iPhone 5, where it was really needed.
When you buy a new MBA or MBP that has MagSafe 2, it comes with a MagSafe 2 charger. The only place I can see the adapted being necessary is if you use a Cinema or Thunderbolt Display. (And if, like me, you have that much $ tied up in a great display, you won't mind spending $10 on one of the cheapest adapters Apple has ever sold!)